Electrical vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs), cumulatively referred to herein as plug-in electric vehicles (PEVs), are vehicles propelled by electricity, as opposed to the conventional vehicles which operate on organically based fuel. The plug-in electric vehicles are composed of an energy storage sub-system (ESS), and an Inverter followed by a Propulsion machine for electric propulsion, called the powertrain.
Electric vehicles operate with higher energy conversion efficiency, produce a lower level of exhaust emissions, and lower levels of acoustic noise and vibration, than conventional vehicles. The electricity needed for electric vehicle operation can be produced either external the vehicle and stored in the ESS, or can be produced onboard with the help of the energy storage source(s) contained in the ESS.
In electric vehicles (EVs), the main energy source can be assisted by one or more energy storage devices. A combination of batteries and super-capacitors are often used as energy storage sources which can be connected to a fuel cell stack in a number of ways. The voltage characteristics of the two devices must match perfectly, and only a fraction of the range of operation of the energy storage devices can be utilized. For example, in a fuel cell/battery configuration, the fuel cell must provide substantially constant power due to the fixed voltage of the battery, and in a battery/supercapacitor configuration, only a fraction of the energy exchange capability of the supercapacitor can be used.
Battery chargers replenish the energy used by an electric vehicle much like a gasoline pump refuels a gas tank. A plug-in electric vehicle (PEV) can be recharged from an external source of electricity, such as, for example, a wall socket. The electricity stored in the rechargeable battery packs drives or contributes to driving the wheels.
The battery charger is a device which converts the alternating current distributed by electric utilities to the direct current needed to recharge a battery. There are a number of different types of battery chargers based on the way they control the charging rate.
Chargers are also classified by the level of power they can provide to the battery pack:
Level 1, which is a common household type of circuit, rated to 120 V/AC and to 15 amps. Level 1 chargers use the standard household three-prong connection and are usually considered portable equipment.
Level 2—permanently wired electric vehicle supply equipment used specifically for electric vehicle charging and is rated up to 240 V/AC, up to 60 amps, and up to 14.4 KW.
Level 3—permanently wired electric vehicle supply equipment used specially for electric vehicles charging and it is rated greater than 14.4 KW. Fast chargers are rated as level 3 chargers.
Electric vehicle battery chargers may be onboard (residing in the electric vehicle) or off board (at a fixed location outside the vehicle). There are two basic coupling methods used to complete the connection between the utility power grid, the battery charger, and the vehicle connector. The first is a traditional plug (the conductive coupling). With this connection, the EV operator plugs his/her vehicle into the appropriate outlet (i.e., 110V or 220V) to begin charging. This type of coupling can be used with a charger in the car (onboard) or out of the car (off board).
The second type of coupling is called inductive coupling. This type of coupling uses a paddle which fits into a socket on the car. Rather than transferring the power by a direct wire connection, power is transferred by induction via a magnetic coupling between the windings of two separate coils, one in the paddle, and the other mounted in the vehicle.
As with many options, there are advantages and disadvantages with both onboard and off-board charger systems. If the battery charger is onboard, the batteries can be recharged anywhere there is an electric outlet. The drawback with onboard chargers is the limitation in their power output due to size and weight restrictions dictated by the vehicle design. Onboard chargers are limited in their power output only by the ability of the onboard charger to deliver the charge. The time it takes to recharge the batteries can be shortened by using a high power off board charger.
Thus, onboard chargers provide flexibility of batteries charging using single-phase power outlets. However, they contribute to additional weight, volume and cost of the car. Due to their charging power limitations and slow charging process, it would take between 4 to 20 hours to fully charge a PEV battery using conventional level-1 and level-2 onboard chargers.
Thus, a high-power charger which does not need additional bulky onboard or off-board power electronic interfaces (PEIs), and which would provide fast onboard charging, without an additional cost and weight, would be highly desirable in the PEVs industry.
The onboard and off-board PEIs for a conventional PEV are illustrated in FIG. 1.
Typically, an onboard charger 10 consists of two stages: (1) an AC-DC stage 12 for rectification of the AC voltage from the grid and Power Factor Correction (PFC), and (2) a DC-DC stage 14 for battery current and voltage regulation.
As shown in FIG. 1, in the onboard power electronic converter of a PEV 18, the conventional onboard battery charger 10 operates independent of the Propulsion machine 20 and the Propulsion Inverter 22. This structural approach is detrimental due to addition of extra components, weight and cost of the vehicle design.
In order to reduce the size, weight and cost of the onboard chargers, different integrated chargers have been designed. For example, initial studies and efforts have focused on integrated non-isolated single-stage chargers that combine an AC-DC rectifier with the Propulsion machine being a DC-DC bidirectional converter (as for example presented in Y. I. Lee, et al., “Advanced Integrated Bidirectional AC/DC and DC/DC Converter for Plug-In Hybrid Electric Vehicles,” IEEE Trans. Vehicular Technology, vol. 58, no. 8, pp. 3970-3980, October 2009).
In comparison to two-stage converters, these single-stage topologies advantageously reduce the number and size of bulky passive components, such as inductors. However, to achieve all the required modes of operation, additional transistors and diodes are needed that increases the complexity and brings to the fore reliability issues. In addition, the propulsion machine DC-DC bidirectional converter is usually rated for much larger power level than an onboard charger. Therefore, utilizing an integrated high-power converter as a low-power converter during relatively low-power charging prevents the attainment of high efficiency operation.
Another group of efforts has been focused on integrated chargers using propulsion machine windings and its inverter (as presented in S. Haghbin, et al., “Grid-Connected Integrated Battery Chargers in Vehicle Applications: Review and New Solution,” IEEE Trans. Industrial Electronics, vol. 60, no. 2, pp. 459-473, February 2013; U.S. Pat. Nos. 4,920,475, 5,099,186, 5,341,075, and W. E. Rippel, et al., “Integrated Motor Drive and Recharge System”). However, the proposed topologies have drawbacks in terms of requirement for bulky add-on components, customization of propulsion machines, access to inaccessible points of the propulsion machine's windings, intensive winding/inverter rearrangement, or the propulsion machine unwanted rotation during charging.
In a typical three-phase PEV propulsion system, shown in FIG. 1, a bidirectional three-phase inverter 22 enables the power flow from the battery 24 to the AC Propulsion machine 20 during the propulsion mode of operation, and the battery charging during the regenerative braking. In order to acquire high driving efficiency, the voltage on the DC_link 26 of the Propulsion Inverter 22 (typically 360 V or 720 V) is preferred to be higher than the rated peak voltage of the AC Propulsion machine 20.
Furthermore, a DC-DC bi-directional converter 28 can be used to regulate DC_link voltage with a wide range of battery voltage variations which adds to the unwanted weight and cost of the existing onboard chargers. DC-to-DC converters are used to interface the elements in the electric powertrain by boosting or chopping the voltage levels as required by the load in different regions of the EV operations. By introducing the DC-to-DC converters, the voltage variations of the devices can be selected, and the power of each device can be controlled.
Another shortcoming of the conventional PEV powertrains is that the only external accessible points to the AC Propulsion machine are the three phase-terminals of the AC Propulsion machine.
It is desirable to provide an innovative solution for high-power onboard charging of PEV batteries which alleviates the above-mentioned limitations of prior technologies.